Arm skeleton. Illustration of the bones of the arm, showing their position in supination and pronation. In supination (left) the palm faces upwards, in pronation (right) it faces downwards. The bones shown are the humerus of the upper arm (pink, at top) and the radius (yellow) and ulna (pink) of the lower arm. At bottom are the carpals (round) and the metacarpals (long) of the wrist and hand. It is the movement of the joint made by the radius and ulna (radio-ulnar joint) that causes supination and pronation. In supination the radius lies lateral to and parallel with the ulna. In pronation, the radius is turned obliquely in front of the ulna.